Signal amplifiers are used in a variety of fields by adopting circuit systems suited for respective fields.
The following descriptions will provide examples of an image display device of an active-matrix drive system and a signal amplifier used in each example will be explained. However, this invention is not limited to the undermentioned examples.
In the following example of the image display device of the active-matrix drive system, a buffer amplifier provided therein will be explained.
As shown in FIG. 30, the image display device of the active-matrix drive system is composed of drive circuits 101 and 102 and a display 103.
The display 103 is composed of display cells C.sub.ij arranged so as to form a matrix. The drive circuit 101 samples a video signal so as to send a voltage obtained as data by sampling over a data signal line S.sub.jp. The drive circuit 102 selects a scanning signal line G.sub.i in order so as to send data on the data signal line S.sub.j to each display cell C.sub.ij. As shown in FIG. 31, in a liquid crystal display, each display cell C.sub.ij is composed of a liquid crystal element 104 and a FET 105 (field effect transistor) for driving the liquid crystal element 104. The display cell C.sub.ij may include a condenser 106 if necessary.
A gate of the FET 105 is connected to the scanning signal line G.sub.i, and a drain and a source of the FET 105 are respectively connected to the data signal line S.sub.j and an electrode of the liquid crystal element 104. The other electrode of the liquid crystal element 104 is connected to a line used in common for all the display cells C.sub.ij. As a result, a transmittance or a reflectance of the liquid crystal of the liquid crystal element 104 are modulated based on data, thereby display an image.
The drive circuit 101 has the following two types: a digital driver for sending digital data over the data signal line S.sub.j ; and an analog driver for sending analog data over the data signal line S.sub.j.
As an example of digital drivers, a digital driver for a 3 bit-input, i.e., an 8-gradation (=2.sup.3) display is shown in FIG. 32.
The digital driver is composed of a shift register 107, a plurality of latches 108, a plurality of switching circuits 109 and a plurality of digital buffers 110.
As shown in FIG. 33, each digital buffer 110 is composed of a decoder 111 for decoding a signal from the switching circuits 109 and a switching circuit 112 for selecting a voltage from V1 through V8 in order to achieve the 8-gradation display according to an output from the decoder 111.
As described, the digital driver requires voltage levels and switching circuits 112 in the same number as the number of gradations. Therefore, in the multiple gradation display, the circuit becomes larger in size. For this reason, the following analog driver is more suited for the multiple gradation display.
The drive system for analog drivers is classified into the following two systems: dot sequential drive system and line-sequential drive system.
As shown in FIG. 34, an analog driver of the dot sequential drive system is composed of a shift register 107, a plurality of latches 108 and a plurality of switching circuits 112. The analog driver outputs data over the data signal line S.sub.j by opening and closing the switching circuits 112 in synchronous with a pulse from each block of the shift register 107.
A period in which an output of data over the data signal line S.sub.j is permitted is H1/N where H1 is an effective horizontal scanning period (approximately 80% of the horizontal scanning period) and N is a number of picture elements in a horizontal direction, i.e., a number of display cells C.sub.i1 to .sub.CiN. For this reason, when displaying an image on a large screen, a period for sending data to the data signal line S.sub.j becomes short.
As shown in FIG. 35, an analog driver of the line-sequential drive system is composed of a shift driver 107, a plurality of latches 108, a plurality of switching circuits 113, a plurality of condensers 116, a plurality of switching circuits 114, a plurality of condensers 117 and a plurality of buffer amplifiers 115 having the previously described characteristics.
The buffer amplifier 115 is a kind of a signal amplifier, and has characteristics of high input impedance and low output impedance and of a voltage gain of substantially one.
Various kinds of circuits are applicable to the buffer amplifier 115 as shown in FIG. 36 through FIG. 38. In the line-sequential drive system, data from the switching circuit 113 is temporarily held in a sampling-use condenser 116 of a small capacity, and the switching circuits 114 are then set ON by a transfer signal in the next horizontal blanking period. As a result, all the data sent in the previous horizontal scanning period are sent into the respective holding condensers 117 and the buffer amplifiers 115 at one time, and are sent over the data signal line S.sub.j. Therefore, a sufficient length of the period is ensured for sending data over the data signal line S.sub.j.
In the described liquid crystal display device of the active-matrix drive system, the FET 105 of the display cell C.sub.ij (see FIG. 13) is formed using an amorphous silicon thin film formed on a transparent substrate, and drive circuits 101 and 102 are formed as separately provided ICs (integrated circuits). Recently, a monolithic system has been reported as a technique for forming the drive circuits 101 and 102 and the display 103 on a polycrystalline silicon thin film.
For the substrate, a quarts substrate or a resin substrate may be used. In the future, it is possible that a glass substrate is used.
The drive circuit 101 of the data signal line S.sub.j once stores a video signal in the sampling condenser 116, and sends it to the buffer amplifier 115. In this state, as the capacitance is divided between the sampling condenser 116 and the hold condenser 117 of the input section of the buffer amplifier 115, a signal (charge amount) decreases. Therefore, when it is necessary to compensate for this reduction, a signal amplifier having a gain of not less than 1 is effective.
FIG. 39 shows a structure of a non-inverting amplifier as an example of the signal amplifier having a gain of not less than 1. FIG. 40 shows an example of a circuit structure of the signal amplifier.
This signal amplifier is a non-inverting type, and the signal amplifier includes an operational amplifier 151, a resistance element 153a connected across an output terminal and an input terminal of the operational amplifier 151 and a resistance element 154b connected across the input terminal and the signal input source, and the gain (ratio of the output voltage to the input voltage) is (Rf/RS)/Rs. Here, respective resistance value of the resistance elements 152a and 152b are denoted as Rf and RS.
In such signal amplifier, a gain can be obtained as desired by setting the resistance value of the resistance elements 152a and 152b or the resistance value of the resistance elements 153a and 153b to appropriate values. However, in such conventional signal amplifier as the buffer amplifier 115, an offset voltage may be generated.
for example, in the polycrystalline silicone thin film transistor, since a particle diameter of a liquid crystal and a channel length of the transistor are in the same order, the characteristic such as the threshold voltage, the mutual conductance, a subthreshold factor, etc., differ depending on a transistor. In the case of adopting the signal amplifier composed of a transistor (polycrystalline silicon thin film transistor, etc.,) having a variable characteristic, an offset voltage may be generated from the signal amplifier. Here, the offset voltage defined as a difference in voltage between the input terminal and the output terminal of the signal amplifier, and in the curve representing the input-output characteristics of the signal amplifier, it is shown as the amount of parallel displacement (the curve does not go through the origin).
For example, in the signal amplifier shown in FIG. 40, the transistors TR4a and TR4b and the transistors TR4c and TR4d respectively make pairs. Therefore, if a difference in characteristic of the transistors in the pair is generated, an offset voltage may be generated in the output from the signal amplifier according to the degree of the difference. Furthermore, a more serious problem arises in that in a circuit including a plural signal amplifiers such as a data signal line drive circuit 101 (FIG. 30), each signal amplifier has random value offset voltage due to a difference in characteristic.
Although such offset voltage is generated also in the driving-use IC formed on a conventional monocrystal silicone substrate, as the level of the offset voltage is suppressed to be not more than several mV, it would not be a problem. In contrast, in the described polycrystalline silicon thin film transistor, since a fluctuation in characteristic is large, an offset voltage of 1 V or above may be generated. For example, when the drive voltage (dynamic range) of the liquid crystal is 5V, if a difference in offset voltage of 1V is generated, it would be impossible to display of 4 or above gradations.
Furthermore, when forming the signal amplifier (FIG. 39 or FIG. 41) having a gain of not less than 1, by a technique of the polycrystalline silicon thin film transistor, fluctuations of the resistance elements 152a and 152b (or resistance elements 153a, 153) would be a problem. Namely, normally, the resistance elements 152a and 152b are formed using the same semiconductor thin film as the transistor, the resistances Rf and RS fluctuate to a large degree, resulting the problem that a gain differs for each signal amplifier. This appears in the curve representing the input-output characteristic of the signal amplifier as a fluctuation in tilt.
To solve the above-mentioned problem, a liquid crystal display disclosed in Japanese Laid-Open patent publication No. 142591/1992 (Tokukaihei 4-142591) discloses a liquid crystal display device wherein a voltage for compensating for an offset voltage in each signal amplifier is stored beforehand as compensating data in a memory, and by inputting a signal obtained by adding this compensation data and the video signal is inputted in the data signal line drive circuit to eliminate the offset voltage, thereby preventing the effect from fluctuation.
In the described method, however, since an offset amount differs for each data signal line as explained earlier, voltage offsets generated from all the signal line outputs are measured after completing the device, and the results of the measurement are stored in the memory. This causes an increase in cost for measuring the device and writing in the memory, and also causes an increase in process cost for integrating a nonvolatile memory into the driving IC.
In addition, the Fermi level and the carriage mobility of the semiconductor vary to a large degree by temperature, and this cause the properties of the signal amplifier to vary by the operation environment temperature of the image display device. In the case where the property of the transistor characteristic fluctuate to a large degree such as the polycrystalline silicon thin film transistor, its temperature dependency may also fluctuate.
Furthermore, in the polycrystalline silicone thin film transistor, plural localized levels exit on a grain boundary of the polycrystalline silicone and an interface between the gate insulating film-polycrystalline silicone interface, and the TFT (thin film transistor) substrate is in a floating potential state. Thus, characteristics of the transistor fluctuate as time passes to a greater degree than the transistor formed on the monocrystal silicone substrate, and this may cause the characteristics (offset voltage and the tilt of the input-output characteristic) of the signal amplifier to fluctuate as time passes.
The described problems associated with fluctuates in temperature and fluctuations as time passes would not be solved by the conventional method (Japanese Laid-Open Patent Publication No. 142591/1992 (Tokukaihei 4-142591)) for storing the offset voltage of each signal amplifier in the memory because, in practice, it is not possible for the user of the image display device to measure the offset voltage of each signal amplifier and to rewrite the content of the memory according to the result.
As described, in the signal amplifier, an offset voltage and a tilt of the input-output characteristic fluctuate, and especially when adopting the signal amplifier composed of polycrystalline silicone thin film, this variation would become large. Furthermore, in the image display device provided with the signal amplifier having such fluctuations, since the offset voltage and the tilt of the input-output characteristic differ for each signal amplifier, it is difficult to achieve a high quality image display.